1. Field of the Invention
Apparatuses consistent with the present invention relate to a liquid crystal display (LCD) device that is switchable between a reflective mode using external light and a transmissive mode using a backlight, by employing an active reflective polarizer.
2. Description of the Related Art
Recently, as the supply of portable devices such as portable phones, personal digital assistants (PDAs), and the like has increased, display devices which can operate with low power consumption and have excellent viewing characteristics in outdoor environments are required. To meet these requirements, research on transflective LCDs having a combination of a transmissive LCD using a backlight and a reflective LCD using external ambient light has been actively conducted.
FIG. 1 is a schematic view of a transflective LCD device disclosed in U.S. Pat. No. 6,654,087. Referring to FIG. 1, the transflective LCD device includes a backlight device 20, a lower substrate 40, a liquid crystal layer 60, and an upper substrate 50. The liquid crystal layer 60 is divided into a region that reflects external light by a reflective electrode 46 included therein, and a transmissive region 43 which transmits light from the backlight device 20.
In such a configuration, half of each cell is used in a transmissive mode, and the other half of each cell is used in a reflective mode. Therefore, the resolution is reduced, and each of the modes has a reduced luminance. In addition, the reflective electrode 46 has to be included inside of liquid crystal cells, and different cell gaps, such as those satisfying the relation of d4=2d3, must be used in order to compensate for optical path differences of the transmissive mode and the reflective mode. Therefore, the manufacturing process of the transflective LCD device is complex.
FIGS. 2A and 2B are schematic views of an LCD device disclosed in U.S. Pat. No. 6,710,831. Referring to the drawings, the LCD device includes a backlight 80, a mirror assembly 70 that includes a Cholesteric liquid crystal (CLC) cell 72 and a quarter wavelength plate 74, a second polarizer 67, a liquid crystal cell 65, and a first polarizer 63. In a reflective mode using external light 61 (FIG. 2A), the CLC cell 72 functions as a mirror that selectively reflects circularly polarized light. Among the unpolarized external light 61, linearly polarized light which is transmitted through the first polarizer 63, the liquid crystal cell 65, and the second polarizer 67 is incident on the CLC cell 72 in a predetermined state of circularly polarized light due to passing through the quarter wavelength plate 74, and thus is reflected by the CLC cell 72. The reflected light is incident on the liquid crystal cell 65 in a linearly polarized light state via the quarter wavelength plate 74, and on/off states are controlled based on a voltage applied to the liquid crystal cell 65 to form images. In a transmissive mode using light from the backlight 80 (FIG. 2B), the CLC cell 72 operates to transmit all of the incident light. Unpolarized light irradiated from the backlight 80 is incident on the liquid crystal cell 65 in a linearly polarized light state via the mirror assembly 70 and the second polarizer 67, and on/off states are controlled based on a voltage applied to the liquid crystal cell 65 to form images.
In such a configuration, the properties of the CLC cell are sensitive to the angle of incidence or the wavelength of incident light, so that in order to obtain reflectance properties with respect to a total visible light region, multi-layers are needed. In addition, a glass substrate used to form a CLC cell and a quarter wavelength plate used for linear polarization conversion are required. Therefore, there are limits to reducing the thickness and manufacturing costs of LCD devices. In addition, the CLC cell has a high turn-on voltage, so that there are also limits in reducing the power consumption.